I am not totally sure, but as far as I know, there is no such message in MavLink and it seems like every application like MP, QGC taranis etc. have their own timer. i.e when they get a MavLink message like arm or set home or another event that indicates takeoff or something like that, they reset this timer to 0 and than simply present it as flight time. The message you have in the taranis is probably from taranis and not from the FC.
And I truly wonder why there is no such functionality in MavLink?
Just my 2C